The invention is directed to improvements in piezoelectric actuators, in particular for actuating control valves or injection valves in internal combustion engines in motor vehicles, having a piezoelectric actuator body, in particular in the form of a multilayer laminate of layered plies of piezoelectric material and intervening metal or electrically conductive layers acting as electrodes, these electrode layers being contacted by electrically conductive common electrode leads, and the actuator body is surrounded by a metal module wall defining an interstice therebetween that contains the common electrode leads.
One such piezoelectric actuator disclosed in German Patent Disclosure DE 196 50 900 A1 of Robert Bosch GmbH.
As is well known, piezoelectric actuators can for instance be used for injection valves of a vehicle motor and in brake systems with anti-lock and traction control systems.
Such injection valves equipped with piezoelectric actuators have an injection nozzle controlled by a tappetlike closure device. An operative face toward the nozzle is disposed on the tappet and is acted upon by the pressure of the fuel supplied to the nozzle; the pressure forces seek to urge the tappet in the opening direction of the closure device. The tappet protrudes with a plungerlike end, whose cross section is larger than the aforementioned operative face, into a control chamber. The pressure effective there seeks to urge the tappet in the closing direction of the closure device. The control chamber communicates with the fuel supply, which is at a high pressure, via an inlet throttle and with a fuel return line that has only low pressure, via an outlet valve that is throttled as a rule or is combined with an outlet throttle. When the outlet valve is closed, a high pressure prevails in the control chamber, by which the tappet is moved in the closing direction of the closure device, counter to the pressure on its operative face toward the nozzle, or is kept in the closing position. Upon opening of the outlet valve, the pressure in the control chamber drops; the magnitude of the drop in pressure is determined by the size of the inlet throttle and by the throttle resistance of the opened outlet valve, or the outlet throttle combined with it. As a result, the pressure in the control chamber decreases when the outlet valve is opened, in such a way that the tappet is moved in the opening direction of the closure device, or held in the open position, by the pressure forces that are operative on its operative face toward the nozzle.
In comparison with electromagnetically actuated injection valves, piezoelectric actuators can switch faster. However, in the design of a piezoelectric actuator, it must be noted that internal losses in the piezoelectric body of the actuator cause lost heat, which has to be dissipated so that the actuator will not overheat. Since the ceramic materials of the piezoelectric ceramic have poorer heat conductivity, the dissipation inside the actuator body, which substantially comprises ceramic material, is unfavorable, especially in long actuators, whose length is greater than their width.
Because of the electrodes that are located in the active part of the actuator body, its heat conductivity crosswise to the electrode layers is higher by a factor of three to five than at right angles to this, since the piezoelectric ceramic material is a poor heat conductor. Naturally, this factor depends on the geometric conditions, such as the thickness of is the ceramic layers, the thickness of the electrode layers, and on the thickness of the non-continuous electrode from the edge of the actuator body. Since the convection, that is, the output of heat to the air, is poor, the greatest part of the heat must be dissipated via the end faces. Since the thermal conduction, as noted, is poor perpendicular to the electrode layers, the heat takes the following course, in simplified terms. It is guided by the internal electrodes of the active part of the actuator body to the outer electrodes. There, it is distributed relatively well and thus quickly reaches the vicinity of the face ends of the actuator body. Since the outer electrodes have no contact with the metal actuator base or the metal retaining plate in the region of the actuator head, the heat has to flow via the poorly conducting face ends of the actuator body. Since the outer electrodes are located on the outside, the heat also flows to the end face only in the outer regions of the actuator body.
Cooling the actuator with a liquid coolant, such as fuel, water, motor oil and the like, which is theoretically possible, is unfavorable, first because of the risk of a short circuit from the water component that is contained both in the fuel and in motor oil, and second because the actuator module is more expensive because of complicated seals, which must preclude the coolant used from escaping from the actuator module, especially when the actuator becomes heated.